1. Field of the Invention
The present invention relates to an orthogonally multiplexed parallel data transmission system for transmitting digital data via a plurality of channels wherein the orthogonalities of the channels must be strictly maintained at all times, and more specifically to (a) a method of synchronizing parallel channels of such a transmission system and (b) an automatic equalizer which features extremely rapid restoration of normal operation in the presence of excessive timing difference after being initially operated.
2. Description of the Prior Art
It is known in the art that an orthogonally multiplexed parallel data transmission system allows spectrum overlapping within a predetermined bandwidth, and hence attains a very high efficiency of data transmission up to approximately the efficiency of the ideal Nyquist transmission. Therefore, such a transmission system has found demands in systems wherein very high efficiencies of digital data transmission are important.
Such a data transmission system, however, requires that the parallel channel synchronization should be establised within a short time after the system is initially operated. To this end, the following two methods have been proposed.
A first known method is to insert a marker symbol into each channel at a predetermined time interval when transmitting information. A receiver detects the marker symbols and adjusts the time points of the markers detected in order to synchronize the parallel channels of the system. This method, however, has been encountered a problem that the insertion of the markers undesirably lowers the efficiency of information transmission.
According to a second known method, a receiver is arranged to previously store reference parallel symbol patterns. Thereafter, the same symbol patterns as prestored in the receiver are transmitted, prior to an actual data transmission, as training signals. At the receiver, the reference symbol patterns applied thereto, are compared with the reference signals previously stored therein in order to specify the transmission line characteristics, after which the parallel channels are synchronized using the inverse characteristics of the transmission line. This second method however has encountered the following problems: (a) the receiver should include a large capacity memory for prestoring the reference parallel symbol patterns and (b) complicated calculations are required to obtain the transmission line characteristics and the inverse characteristics thereof.
Further, this second method has another difficulty that a large eye-opening is not obtained due to a limited frequency resolution. More specifically, assuming that (a) the transmission bandwidth is "B" and (b) the number of the parallel channels is "N", then the frequency resolution is represented by B/N which is, in the case of the quadrature multiplexing system, approximately equal to a baud rate (f.sub.0) of each channel. This means that a frequency resolution of the second method is defined by an interval of f.sub.0. However, this value is not sufficient in that the frequency resolution in the order of f.sub.0 /10 is required for a large eye-opening of each channel. In order to overcome this problem, further complex calculations are needed with the second known method, although not mentioned for simplicity. For further details of the second method, reference should be had to "Frequency Domain Data Transmission using Reduced Computational Complexity algorithms", 1980 IEEE, pp. 964-967.
Further, there has been another problem with the above type of conventional system in that in the presence of excessive timing differences when initially operated, the automatic equalizers used therein are very slow in establishing normal operations thereof.